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    <script>
        //变量基本声明
        // es5
        var num = 10;
        var num = "abc";

        // es6
        let a = 10; //变量
        //let a = 30; //变量不能重复声明


        const pi = 3.14; //常量
        pi = 4;
        console.info(pi);
    </script>
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